1. Technical Field
Aspects of this document relate generally to photovoltaic systems.
2. Background Art
FIG. 1 describes a typical photovoltaic (PV) grid-tied 100 or off-grid 110 system. A PV system consists of a number of modules 101; each module by itself generates power when exposed to light. A series of modules is wired together to create a higher voltage string 102. Multiple PV strings may be wired in parallel to form a PV array 103. The PV array connects to a DC-disconnect switch 104, and the DC disconnect switch feeds power to a grid-tied inverter 105 which converts the DC power from the array to AC power for the grid.
Off-grid systems 110 connect the PV array 103 to the DC disconnect, and on to a battery charger 111, which stores the electrical energy in batteries 112. Off-grid residential systems typically use an off-grid inverter 113 that produces AC electricity for AC loads connected to an AC main panelboard 106.
Inside a silicon cell based module 200, shown in FIG. 2, there is a series of photovoltaic cells 201, the basic building block in solar electric systems. Each cell is producing approximately 0.5 volts and a few amps (e.g. 5 A). The PV cells are also wired in series and in parallel within the module to achieve a desired voltage and current, and each module has a positive and negative module terminal 202 to connect to the PV system. A typical module used in a residential or commercial power generating system will produce in the order of 18-50V DC at 90-200 W at its electrical connectors. There are two terminals one positive and the other negative. Arrays used in residential installations will typically produce power in the range of 2 kW-10 kW with voltages up to 600V DC (grid-tied). The module voltage and power output is true for other module architectures such as thin-film (CdTe, CIGS, etc.)
When a PV array is installed and operational, the PV system generates power whenever there is light present. Furthermore, it is impractical to disable the system beyond shutting off the AC mains or the DC disconnect. Once wired, the array itself is never able to fully shut down in the presence of light even with the DC disconnect in the open position. The string wiring connecting all the modules in series, the wiring to the DC disconnect, and the array will all continue to generate lethal levels of voltage when exposed to light.
In the case of a damaged array from fire or natural disaster, an open (non-insulated) wire of the array's circuits may present itself. The exposed circuits provide a higher likelihood of an unintended electrical circuit path to ground (ground fault), and a human can become a part of this path to ground either by touching or through exposure to water. With a human body in a ground fault circuit it is very likely to be lethal. The National Fire Protection Association (NFPA) 70E defines “low voltage” somewhere near ˜50V. This low voltage is the threshold where one is able to generally survive a shock and “let go” (˜9 mA). PV systems are well above this level. This poses a serious and very real problem for firefighters when they encounter a building on fire with a PV array.
Even an operational and properly insulated system poses a potential problem for service technicians in the case of a PV array in need of service. In the case of the need to replace a defective module the person may be exposed to high voltages even with the DC disconnect in the “off” or “open” position.
In the case of earthquakes, floods, or other natural disasters, partially destroyed PV systems pose a threat to the occupants of a structure and any rescue personnel, especially untrained civilians.